Friday, August 8, 2014 3:00PM (334 JFB)
Title: Origin of Coloration in Beetle Scales: An Optical Structural Investigation
In this thesis the origin of angle-independent yellowish-green coloration of the exoskeleton of a beetle was studied. The beetle chosen was a weevil with the Latin name Eupholus chevrolati. The origin of this weevil’s coloration was investigated by optical and structural characterization techniques combined with three-dimensional modeling and photonic band structure calculations. Furthermore, using color theory the pixel-like coloring of the weevil’s exoskeleton was investigated and an interesting additive color mixing scheme was discovered.
For optical studies, a micro-reflectance microscopy/spectroscopy set-up was optimized, which allowed not only for imaging of individual colored exoskeleton domains with sizes ~2-10 µm, but also for obtaining reflection spectra of these micrometer-sized domains. Spectra were analyzed in terms of reflection intensity and wavelength position and shape of the reflection features. To find the origin of these colored exoskeleton spots, a combination of focused ion beam milling and scanning electron microscopy imaging was employed. A three-dimensional photonic crystal in form of a face-centered cubic lattice of ABC-stacked air cylinders in a biopolymeric cuticle matrix was discovered. Photonic band structure calculations revealed the existence of different sets of stop-gaps for the lattice constant of 360, 380 and 400 nm in the main lattice directions, Γ-L, Γ-X, Γ-U, Γ-W and Γ-K. It was also discovered that irrespective of the lattice constant value, longer wavelengths (reds) were generated only from the Γ-L direction, whereas the shorter ones (violets) were forbidden to enter the crystal in the Γ-W and Γ-K directions.
In addition, scanning electron microscopy images were compared to the specific directional-cuts through the constructed face-centered cubic lattice-based model and the optical micrographs of individual domains to determine the photonic structure corresponding to the different lattice directions. The three-dimensional model revealed stop-gaps in the Γ-L, Γ-W and Γ-K directions.
Finally, the coloration of the weevil as perceived by an unaided human eye was represented (mathematically) on the xy-chromaticity diagram, based on XYZ color space developed by CIE (Commission Internationale de l’Eclairage), using the micro-reflectance spectroscopy measurements. The results confirmed the additive mixing of various colors produced by differently oriented photonic crystal domains present in the weevil’s exoskeleton scales, as a reason for the angle-independent dull yellowish-green coloration of the weevil E. chevrolati.